1. Field of the Invention
This invention relates to a transparent sheet having a heat-transferred image thereon which is suitable to use in an overhead projector (referred to hereinafter as OHP) in which copying is effected by a heat transfer system printer.
2. Prior Art
In general, a transparent sheet for forming an image thereon by a heat transfer system and for use in the OHP is a stretched polyester sheet, the surface of which has been subjected to antistatic treatment or treatment for improving the toner-adherability, and images such as letters, figures, patterns and the like are copied on the sheet by means of a copying machine, and the sheet having images thus formed is used in the OHP.
In a copying machine, information is transferred with a molten toner onto a transparent sheet, so that the sheet curls or wrinkles in some cases. Also, when the toner does not sufficiently adhere to the sheet, the sheet is smudged and the information cannot exactly be transferred. With a conventional polyester sheet, a rubber-based adhesive layer, an acrylic resin adhesive layer or the like is coated on the sheet in order to improve the toner-adherability. However, even if such a treatment is applied to the sheet, the thus treated sheet is not necessarily sufficient in toner-adherability. Such adhesive layers impair the transparency of sheets for the OHP, and the sheet having coated thereon the adhesive layer has an disadvantage that when such sheets are subjected to the OHP the resulting screen becomes cloudy and indistinct.
Recently, use of color copies has been started, and in a color copying machine, three-color toners are successively fed onto a transparent sheet to form a color image at a temperature higher than in a black-and-white copying machine, so that the time for which the sheet is kept at such higher temperatures becomes longer. Under such high temperature conditions, conventional stretched polyester sheets become shrunk or softened, and hence, it has more often been caused than in the black-and-white copying that the sheet wrinkles or changes in dimension.
The transparent sheet on which an image has been copied in such a manner is subjected to the OHP to project the image on a screen; however, if the sheet is curled by heat during the copying the end portions of images are faded and the resulting images become indistinct. Also, owing to insufficient toner-adherability, the necessary images become indistinct or the toners are stripped off from the sheet during the repeated use thereof.
In recent years, use of the OHP has been more general than use of a slide in disclosure in academic society meetings and lecture meetings or representation by salesmen. For this purpose, sheets for the OHP are often carried and it has been strongly desired that the sheets are lighter. For lightening, it is necessary to make the specific weight of material for the transparent sheet smaller and make the sheet thinner. However, when a sheet having insufficient heat resistance is made thinner, the resulting thinner sheet has a tendency that the sheet wrinkles deeply during copying and the copying machine is jammed by the sheet.
In order to make the representation more beautiful, there is a strong desire of coloring sheets for the OHP. For this coloration, the sheet materials are required to have excellent compatibility with various dyestuffs, and the transparent sheets per se are required to be excellent in transparency because the coloration adversely affects the light transmissibility of the transparent sheets. Conventional polyester sheets are not always satisfactory in transparency, and cannot be said to be suitable for distinct coloration, resulting in a cloudy screen.
Other uses of the transparent sheets include celluloid pictures for animation. In the conventional celluloid pictures for animation, a triacetyl cellulose (referred to hereinafter as TAC) film is used as a transparent sheet, and the celluloid pictures are prepared by copying the original picture thereon through a carbon paper and coloring the copied picture. Usually, several celluloid picture sheets are used in pile, and therefore, excellent transparency is strongly required. Hence, the TAC film has been used. However, the TAC film is inferior in heat resistance and has been unable to be subjected to copying by a heat transfer system, and therefore, a copy has been made thereon through a carbon paper. In this case, however, the copied image has been uneven and tended to be smudged, and therefore, amendment and correction of the image have been necessary. For simplifying the amendment and correction step, a transparent sheet on which a copy can be made by a heat transfer system has been desired. Conventional polyester sheets are not so sufficient in transparency as to be used to make a celluloid picture for animation, and when several sheets thereof are put one on another the resulting screen is cloudy and no clear image can be obtained. Thus, the conventional polyester sheets cannot be used as sheets for making a celluloid picture.